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There is now overwhelming evidence in the scientific literature that caloric restriction (CR) increases longevity in a variety of organisms including yeast, roundworms, fruit flies, and rodents (see ARF related news story). But does CR have a similar benefit for primates, including humans? Despite the lack of hard evidence, there are some individuals who have gone with their gut—literally—and reduced their caloric intake by up to 30 percent in the hope of postponing the inevitable. So will the sacrifice be worth it?

The jury is still out, but the relationship between caloric restriction and human health was one of the major themes in Oakland last weekend at the American Aging Association annual meeting, organized by association president Andrzej Bartke, University of Southern Illinois, and colleagues.

In the pre-conference meeting that focused on the impact of nutrition and lifestyle on aging and neurodegenerative disease, Donald Ingram from the National Institute on Aging (NIA), Baltimore, Maryland, reviewed some of his data on the effects of caloric restriction in non-human primates. First, the bad news. The study, initiated by the NIA to address the dearth of data on primates, has failed to detect any statistically significant extension in lifespan in rhesus monkeys, some of which have been on a CR diet since 1987. However, because the average lifespan of a rhesus monkey is about 25 years (maximum lifespan is about 40 years) and the study is only 18 years old, it may be too early to tell if the CR regimen is having any real effect on longevity.

Instead, the silver lining suggests that if the monkeys on the reduced calorie diet are not living longer, they are at least living healthier. For example, the CR animals have much better insulin sensitivity and lower plasma insulin levels—two parameters that have been correlated to longer life in humans. And in many standard tests, such as those that might be encountered in a regular clinical setting, the younger monkeys on the low calorie diet did better. They had lower blood pressure and triglycerides and increased HDL cholesterol, changes that are indicative of healthy living. They also seemed to be protected from some hormonal changes. The gradual increase in plasma follicle stimulating hormone that normally occurs during aging was attenuated in monkeys on the CR diet, as was the decline in plasma dehydroepiandrosterone sulfate (DHEAS). But it was perhaps appearance-wise where the monkeys differed most. Those fed a normal diet looked heavier, greyer, and somewhat tired and worn out compared to their CR-diet counterparts.

On the first day of the meeting proper, data presented by Richard Weindruch, VA Hospital in Madison, Wisconsin, seemed to support Ingram’s findings. Weindruch has been studying the effects of a CR diet on rhesus monkeys since 1989 and has found that levels of fat, C-reactive protein, oxidative damage, and osteoarthritis of the spine are all reduced in animals on reduced calories. Further signs of healthier aging include increases in insulin sensitivity and adiponectin, a cytokine that accentuates insulin signaling. The study is ongoing, slated to be completed in 2020.

But what about humans? Not too long ago, the prospect of carrying out a human study on caloric restriction and aging seemed like nothing more than idle, after-dinner chatter. The general consensus then was that finding sufficient numbers of volunteers who would agree to such a drastic cut in calories (about 30 percent) would be hard enough. But finding volunteers who would stay on the diet would be the real sticking point. Well, enter the CRONies.

John Holloszy, Washington University School of Medicine, reported how with the help of the Calorie Restriction Society he was able to recruit dedicated volunteers who were already practicing caloric restriction with optimal nutrition (CRON). Data collected from this group suggest that those who have been on the CRON regimen for between 3 and 14 years have much healthier physiological and biochemical profiles than do those on regular diets, or even those on regular diets who get plenty of exercise.

The exercise group, for example, had an average body weight of about 70 Kg; that’s about 11 Kg lower than the average sedentary volunteer. But the CRONies weighed in at another 11 Kg lighter, or 59 Kg (that’s about 130 pounds), evidence that they are serious about their CR regimen. The CRONies also had much lower trunk fat (2.4 percent) compared to those in the exercise group (7.5 percent).

The CRONies had lower average LDL cholesterol (91 mg/DL) than did the exercise or control groups (95 and 122 mg/DL, respectively), lower triglycerides (57 mg/DL vs. 65 and 162 mg/DL), and higher HDLs. Blood pressure was also markedly different in the CRON, exercise, and control groups (103/62, mmHg vs. 126/73, and 132/83 mmHg, respectively), as were plasma insulin (1.6, 1.8, and 8.2 μU/mL), glucose, leptin, and other factors that are indicative of less than optimal health. These included C-reactive protein (0.23, 0.73, and 1.27 mg/L) and tumor necrosis factor α (0.8, 1.4, 1.4 pg/mL).

The only downside to the CRON regimen appears to be cold intolerance. These volunteers always complain of being cold, Holloszy said. This may be related to their lower levels of T3 thyroxine, which regulates metabolism. In terms of diastolic function, the CRONies are about 10 years younger than the control group, suggested Holloszy. They also have decreased numbers of white blood cells and other lymphocytes—parameters that are also reduced in animals on CR diets.

Holloszy also reported data from a 1-year intervention study that suggest the benefits of caloric restriction come quickly. In this study, improvements in lipid biology and blood pressure were all seen within a year.

Eric Ravussin’s solution to volunteers straying from their dietary regimen is to accommodate them at the Pennington Biomedical Research Center in Baton Rouge, Louisiana. The center is set up with a “metabolic kitchen” which ensures the diets are of good quality and calorically accurate.

Ravussin reported results of an intervention study designed to assess the benefit of calorie restriction, or a combination of calorie restriction and exercise, on 48 healthy but overweight male and female volunteers who spent the first 3 months and last 2 weeks of a 6-month study at the center.

The volunteers were split into four groups. In one, calories were reduced to 25 percent of that required to keep the subjects' weight stable. The second group was put on mild caloric restriction (12.5 percent of that required to maintain weight), plus enough physical activity to soak up another 12.5 percent of their calories. In a third, a very low-calorie diet (LCD) was designed so that 15 percent of body weight would be rapidly lost, but then the volunteers would be allowed to consume enough calories to maintain that weight. A fourth group, with no intervention, served as controls.

Not unexpectedly, except for the control group, all of these volunteers lost weight. Those in the intervention groups also had lower plasma insulin than did controls (33, 24, and 10 percent lower for the CR, CR plus exercise, and LCD groups, respectively). All the groups demonstrated a 25-30 percent improvement in insulin sensitivity. Though magnetic resonance imaging revealed that there was no difference in levels of muscle fat among the four groups, those on intervention had up to 50 percent lower fat in the liver.

Ravussin and colleagues are also set up to measure cellular parameters that might shed some light on what changes occur during calorie restriction. They found, for example, that spontaneous oxidation of DNA was lower in the three intervention groups compared to controls, though they detected no difference in protein carbonyls, which herald protein oxidation. Ravussin and colleagues also have ongoing microarray analysis of protein expression in muscle and adipose tissue, and they are examining the effects of CR on mitochondrial biogenesis. They found, for example, that levels of the mitochondrial protein PGC1α are increased in the CR and LCD group, but not in the CR plus exercise group. They also found that levels of SIRT1, a protein that has been implicated in longevity in yeast, and mammalian cells (see ARF related news story), are elevated almost twofold in the CR volunteers.

Ravussin reported that volunteers in all the intervention groups had a lower 24-hour energy expenditure than would be expected, even correcting for body weight. This suggests that there has been some metabolic adaptation to the caloric restriction. The basis for this is currently being investigated.

But what about humans who live to ripe old ages? Is there any correlation between longevity as we know it now, and caloric intake? There just might be. Bradley Willcox from the Pacific Health Research Institute, Honolulu, Hawaii, showed some data from the world’s longest-running population-based study on centenarians, the Okinawan Centenarian Study.

The islands of Okinawa are home to the highest concentration of centenarians in the world. The people there also have the longest disability-free life expectancy in the world, reported Willcox. So why is that?

Willcox suggested that there may well be some genetic influence because siblings of Okinawan centenarians generally live longer, too. But he also emphasized the role of diet. Historically, Okinawa has been the most undernourished prefecture in Japan, and data from the Okinawan Centenarian Study supports the theory that reduced caloric intake has contributed to their longevity. Okinawans have had the lowest average body weight of all Japanese, for example, and levels of DHEA, which inversely correlates with caloric intake in animal studies, are much higher in male and female Okinawans than in age-matched Americans, Willcox reported.

However, things are changing in Okinawa. Unfortunately, Okinawans now rank as some of the heaviest Japanese, so unbeknownst to many people from that prefecture, they may be taking part in one of the largest studies to date on caloric intake and longevity. Should Okinawa also lose its standing in the centenarian tables, one might conclude that it is because they are spending more time at the dinner table. Only time will tell.—Tom Fagan.